Light modulator comprising a photochromic layer

ABSTRACT

A light modulator ( 1 ) is proposed which has a photochromic layer ( 15 ), which can be activated optically by control light, for modulating signal light, and at least one optically transparent substrate ( 13 ) for the photochromic layer ( 15 ), in which the light modulator ( 1 ) has at least one filter layer ( 14 ), which reflects the control light in a wavelength-selective fashion for the purpose of retroreflecting control light which has penetrated the photochromic layer ( 15 ). The reflecting filter layer ( 14 ) ensures that the control light is utilized efficiently for the photochemical conversion (photoconversion) of the photochromic material, since the control light passes twice through the photochromic layer ( 15 ). The reflecting filter layer ( 14 ) has the function, furthermore, of separating the control light from the modulated signal light such that the modulated signal light can be evaluated without significant interference from the control light.

DESCRIPTION

The invention relates to a light modulator having a photochromic layer,which can be activated optically by control light, for modulating signallight, and at least one optically transparent substrate for thephotochromic layer.

BACKGROUND OF THE INVENTION

Light modulators of the aforementioned type are also denoted asoptically addressable, spatial light modulators. Although thephotochromic layer is “addressed” optically only two-dimensionally andnot three-dimensionally, it is usual to talk of a spatial lightmodulator instead of a planar one. Such light modulators are denotedbelow as OASLMs.

The photochromic layer serves the purpose of transmitting or conveyinginformation from the control light to the signal light. In the event ofirradiation with control light of a predetermined first opticalwavelength, the photochromic layer reacts at the site of irradiationwith a change in specific optical properties—in particular with a changein the optical absorptivity—for signal light of a predetermined secondoptical wavelength. For example, the control light can be used toproject an intensity contrast image onto the photochromic layer, whichthen reacts with a setting, corresponding to the contrast image, of itsabsorptivity as regards the signal light beyond the area irradiated bythe control light. If the photochromic layer thus activated optically bycontrol light is irradiated with signal light, the signal light emergingfrom the photochromic layer has a modulation corresponding to theabsorption contrast pattern. Information from the control light cantherefore be transmitted onto the signal light in a planar fashion in away which varies with time. The signal light striking the photochromiclayer can be an extended light bundle which simultaneously covers theentire light entry area of the photochromic layer. A correspondingstatement holds for the control light. However, it is also possible to“write” the relevant information into the photochromic layer with theaid of a deflectable control light beam. Likewise, it is also possiblefor the purpose of “reading” or “erasing” the information to make use ofa signal light beam or “erasing light beam” which, for example, scansthe photochromic layer by row or by column.

A multiplicity of photochromic materials come into consideration forsuch applications. An overview of the essential photochromic materialclasses, their best known representatives and their properties is to befound in H. Dürr, H. Bouas-Laurent, “Photochromism—Molecules andSystems”, Studies in Organic Chemistry, Elsevier, Vol. 40, 1990. Inaddition to other photochromic materials such as, for example, syntheticinorganic and organic photochromics, bacteriorhodopsin in the form ofa-purple membrane, denoted below as BR, is a particularly interestingmaterial for forming the photochromic layer. Purple membrane is the formused for the naturally occurring two-dimensional crystalline form ofbacteriorhodopsin. The design of the so-called purple membrane fromlipids and bacteriorhodopsin is described in numerous examples in theliterature. D. Oesterhelt et al., Quart. Rev. Biophys., 24 (1991)425-478 may be cited by way of example as a reference.

It is, inter alia, the following five reasons which renderbacteriorhodopsin particularly suitable for the application outlined.

(i) BR is distinguished by a very efficient photochemical reaction withseveral photoactive states which render it possible to implement“writing” and “erasing” photochemically.

(ii) BR has a particularly high reversibility, and this predisposes itto dynamic use.

(iii) The specific absorptions of the long lived states of BR, and alsothe difference in refractive index between these states are very high,and so good modulation of the signal light is achieved.

(iv) Bacteriorhodopsin has a strongly anisotropic chromophore and istherefore suitable for polarization-selective modulation.

(v) Apart from the wild type of BR, there is currently available a wholeseries of variants of BR produced using gene technology and havingaltered amino acid sequences and/or variants, which contain aschromophores molecules differing chemically from retinylidene radicaland have other spectral and/or other photokinetic properties than thewild type, for example different absorption properties and/orsubstantially longer lived photointermediates.

The material group specified in (v) is denoted below as BR variants. Theterm bacteriorhodopsin or BR is used in such a way that either the wildtype of bacteriorhodopsin or one of the BR variants is understoodthereby. Furthermore, the term bacteriorhodopsin or BR is used both formonomeric BR and for BR in the form of purple membranes. BR variants maybe obtained with various methods. An overview of known methods forproducing mutated bacteriorhodopsins and BR analogs, which are typifiedby the presence of chromophore groups differing from the retinylideneradical occurring in the wild type, is given in N. Vsevolodov,“Biomolecular electronics—an introduction via photosensitive proteins”(1988), Birkhaüser, Boston, Chapter 3. Typical BR variants of technicalinterest which are obtained by modifying the amino acid composition ofwild type BR are those with a lengthened lifetime of the so-called Mstate, for example those in which the aspartic acid at position 96 hasbeen replaced or removed or displaced in its position by removal ofother amino acids, or those with a high probability of the formation of9-cis-retinal, for example those in which the aspartic acid at position85 has been replaced or removed or displaced in its position by removalof other amino acids. Typical technically interesting BR variants whichare produced by replacing the retinylidene radical occurring in wildtype BR by analog molecules are, for example, 4-ketoretinal anddihydroretinal (Sheves et al., Biochem., 24, 1985, 1260-1265). It may bepointed out expressly that a combination of modifications of the aminoacid composition and replacement of the chromophore group is alsounderstood by the term BR variants.

Said possibilities and properties of BR are known to the person skilledin the art and have also influenced applications of BR in variousoptical information processing techniques.

The optically active component is formed by the BR layer in OASLMs. Theoptical modulation is based on the fact that bacteriorhodopsin can beconverted from the initial state B (maximum absorption at approximately570 nm) by irradiation of light of wavelength λ_(B) into at least oneother spectrally different state. The longest lived state of thephotocycle of wild type BR is usually denoted as M state (maximumabsorption at approximately 410 nm). Light of wavelength λ_(M) can beused to convert said state photochemically into the initial state B.Consequently, light in the wavelength region of λ_(B) can be variedand/or controlled, or else vice versa by simultaneous illumination ofthe BR layer with light in the wavelength region of λ_(M), using the BRlayer as a mediator.

The degree of modulation depends in this case on the magnitude of thephotochromic optical absorption changes which is caused by theirradiation of light in the BR layer, on the quantum yield of thephototransformations BM and the intensities and wavelengths of the twooptical irradiations. Because of the polarization-sensitivephotoreaction of the BR, the relative position of the polarizationstates of the two wavelengths or wavelength regions likewise plays arole in the level of the modulation. Furthermore, the local refractionindex, which can likewise be used for modulation purposes, is modulatedin a manner proportional to the absorption modulation.

OASLMs have been known for a long time as active optical components inbeam paths for the purpose of optical processing of images andinformation, and are used to control and/or to modulate the amplitude,the phase and, if appropriate, also the polarization of a spatiallyextended lightwave field as a function of the intensity of a controllight source.

An overview of the state of knowledge concerning BR, and the possibilityof applying BR in optical information processing can be gathered, interalia, from the articles by D. Oesterhelt et al., Quart. Rev. Biophys.,24 (1991) 425-478, D. Zeisel and N. Hampp, J. Phys. Chem., 96 (1992)7788-7792, N. Hampp et al., Proc. SPIE—Int. Soc. Opt. Eng., 1732 (1993)260-270 and N. Vsevolodov, “Biomolecular electronics—an introduction viaphotosensitive proteins” (1998), Birkhaüser, Boston.

The use of a spatial light modulator in a beam path for the purpose ofholographic writing and reading of optical data which are stored in a BRlayer is described in R. R. Birge et al., Ann. Int. Conf. IEEE Eng. Med.Biol. Soc. 12 No. 4, (1990), 1788-1789.

A spatial light modulator which is based on a BR layer and has been usedas a spatial frequency filter for optical image correction, inparticular for optical edge reinforcement is likewise described in thearticle by R. Thoma et al., Opt. Lett. 16 (1991) 651-653.

A specific spatial light- modulator based on a Perot-Fabry resonator,which contains a BR layer as active element, is described in U.S. Pat.No. 5,618,654.

The known light modulator has two plane-parallel, semitransparentmirrors situated opposite and parallel to one another. With the givenmirror spacing L and refractive index n of the medium between themirrors, the Fabry-Perot interferometer corresponding to the resonancecondition L=N λ_(ir)/2 n is virtually completely transparent to light ofwavelength λ_(ir) although, viewed individually, the mirrors must have ahigh reflectivity to light of the resonance wavelength λ_(ir). In thecase of the subject matter of U.S. Pat. No. 5,618,654, the refractiveindex of the photochromic layer between the resonator mirrors is variedby irradiation with control light of wavelength λ_(v) in orderoptionally to fulfill the resonance condition for signal light ofwavelength λ_(ir). In this way, the transmittivity of theinteroferometer light modulator is varied overall for the signal lightλ_(ir), and thus the signal light is modulated. In order to enable thecontrol light required for changing the refractive index to reach thephotochromic layer, in order to function the known modulator requiresthe resonator mirrors to be transparent to the control light with ashigh as possible a transmittivity, and therefore to have as small areflectivity as possible, whereas the reflectivity of the respectiveresonator mirrors to signal light must be as high as possible inaccordance with the functional principle of the Fabry-Perotinterferometer.

Further examples of light modulators which function using the principleof the Fabry-Perot interferometer are described in DE-A 19 35 881 andU.S. Pat. No. 4,834,511. In order to function, all these lightmodulators using the principle of the Fabry-Perot interferometer requireprecise observation of the geometrical relationships, in particular thespacing between the resonator mirrors, set to fulfill the resonancecondition. Maintaining the mirrors in a mutually parallel position, andavoiding fluctuations in the spacing between the mirrors over the entiremodulator area also causes problems. The previously mentionedgeometrical conditions which must necessarily be observed in the knownlight modulators usually require a vibrationless and thermostatic designin the case of light modulators using the interferrometer principle.

Further details on light modulators employing a BR layer follow from thepapers by R. B. Gross et al., Proc. SPIE-Int. Soc. Opt. Eng. 1662 (1992)186-196 and Q. W. Song et al., Opt. Lett. 18 (1994) 1373-1375, and alsoH. Takei and N. Shimizu, Opt. Lett. 19 (1994), 248-250.

SUMMARY OF THE INVENTION

The invention is based on the object of developing an integrated opticalcomponent which is based on an optically addressable, spatial lightmodulator with improved application properties and which can be used ina versatile fashion as an active switching and/or control element inbeam paths for the purpose of optical imaging, in optical displaysystems, in optical systems for information storage and processing, andalso in holographic measuring and processing systems.

Starting from a light modulator of the type mentioned at the beginning,this object is achieved-according to the invention by virtue of the factthat the light modulator has at least one filter layer, which reflectsthe control light in a wavelength-selective fashion, for the purpose ofretroreflecting control light which has penetrated the photochromiclayer the reflecting filter layer having a reflectivity of at least 80%as regards the control light.

The control light can reach the photochromic layer from a control lightentry side of the light modulator, and penetrate into the photochromiclayer. The reflecting filter layer is located on the side of thephotochromic layer averted from the control light entry side, andensures that the control light is retroreflected again to thephotochromic layer. As a result, the control light is utilizedsubstantially more effectively for the photochemical conversion(photoconversion) of the photochromic material, since the control lightpasses twice through the photochromic layer, and the control light pathin the photochromic layer is thereby doubled. In this way, theintensity-dependent degree of modulation of the photochromic layer issubstantially improved. This leads to economic advantages, since it ispossible to use control light sources which are of lower power and thusmore cost-effective. This holds, in particular, for lasers as controllight sources. Alternatively, there is a reduction in the need for BRquantity per area of the OASLM in order to achieve a prescribed degreeof modulation for a given control light source. This yields economicadvantages, since, in particular, genetically altered bacteriorhodopsinsare expensive.

The reflecting filter layer does not, however, have only the function ofefficiently utilizing the control light for optical activation of thephotochromic layer, but also the function of largely separating thecontrol light from the modulated signal light, doing so by passing thesignal light modulated at the photochromic layer toward a light exitside of the light modulator, and reflecting the control light in theopposite direction as determined by the reflectivity. The modulatedsignal light can therefore be utilized by the control light withoutappreciable interference. This point of view is of particularsignificance if a comparatively intensive laser beam is used as controllight beam to “describe” the photochromic layer and visual observationof the photochromic layer is intended to be performed from the signallight exit side of the light modulator, or when a light-sensitivemedium, for example a photosensitive layer, is situated in the signallight beam path downstream of the light modulator.

The light modulator according to the invention has considerableadvantages by contrast with the known light modulators already addressedabove, which function using the principle of the Fabry-Perotinterferometer and have mirrors with as small as possible a reflectivityfor the control light. These advantages include a simple design which iscomparatively uncritical with regard to the dimensions of layers andspacings between the filter layers. Thermostating measures are notrequired with the light modulator according to the invention, sincelinear expansion effects do not appreciably affect the functioning ofthe modulator, and so the functioning of the light modulator accordingto the invention is not impaired by normal temperature fluctuations.Since the thickness of the photochromic layer can to a large extent beselected freely in the case of the light modulator according to theinvention, the production engineering requirements placed on theobservation of tolerances etc., are also slight. The greater degrees offreedom with regard to any possible fluctuations in the thickness of thelayers of the light modulator according to the invention facilitate theimplementation of relatively large light modulator areas.

Again, it is possible to operate with polychromic signal light in thecase of the subject matter of the present invention.

The filter layer reflecting the control light is preferably arrangedbetween the photochromic layer and the substrate, being in directcontact with the photochromic layer. The fact that the photochromiclayer and reflecting filter layer are directly coupled prevents asubstantial beam offset which would reduce the useful resolution of theOASLM. Thus, direct coupling of photochromic layer and reflecting layereconomizes on optical components, and this leads to economic advantagesand minimizes the overall size of the system.

Furthermore, because the interfering control light is removedimmediately downstream of the photochromic layer, the control lightcomponent in the downstream signal light beam path can be substantiallyreduced, and so the signal-noise ratio can be improved. An additionalimprovement in the signal-noise ratio results from the fact that thenumber of the internal interfaces, and thus the reflection losses arereduced.

The reflecting filter layer preferably has a reflectivity of at least99% for the wavelength of maximum reflectivity, and so it is possible toseparate the control light from the modulated signal light virtuallycompletely.

The photochromic layer -preferably contains bacteriorhodopsin as activecomponent.

It is particularly preferred for the photochromic layer to contain avariant of the wild form of the bacteriorhodopsin, which has a higherlight sensitivity and/or a longer service life of the longest livedintermediates than the wild form, and specifically in particular avariant in the case of which the amino acid position 85 is modified, ora variant in the case of which the amino acid position 96 is modified,or a variant in the case of which dihydroretinal or 4-ketoretinal servesas chromophore group, or a variant in the case of which bothdihydroretinal or 4-ketoretinal serve as chromophore group and the aminoacid position 85 and/or 96 are modified.

The OASLM according to the invention can, in particular, have on atleast one side an antireflection layer effective over a wide band ofvisible light.

Furthermore, it can be expedient to apply a protective layer transparentto visible light at least to the side of the photochromic layer avertedfrom the substrate.

In accordance with a development of the invention, it is possible toprovide a second wavelength-selectively reflecting layer with awavelength-selective reflectivity differing from the first reflectingfilter layer. Essentially the same result can also be achieved with acoating when the latter has two or, possibly, even more wavelengthregions in which there is a pronounced selective reflection.

In accordance with a particularly preferred development of theinvention, on the side of the photochromic layer averted from thereflecting filter layer a filter layer which reflects the signal lightin a wavelength-selective fashion the light modulator has andretroreflects signal light which has penetrated the photochromic layer.Such a light modulator having a respective reflection filter on mutuallyopposite sides of the photochromic layer constitutes an opticalcomponent which is suitable for interesting applications such as, forexample, for the incoherent/coherent conversion still to be describedbelow, or for the frequency conversion likewise still to be explained.

For various applications, interest may attach to a light modulator ofthe type mentioned in the beginning which has only one filter layerreflecting the signal light in a wavelength-selective fashion in orderto retroreflect signal light which has penetrated the photochromiclayer. Such a light modulator therefore outputs the modulated signallight to that side at which the unmodulated signal light has entered thelight modulator.

The invention also relates to an optical display device having a lightmodulator as claimed in one of claims 1-6 as display element. Theoptical display device comprises a control light source for activatingthe photochromic layer of the light modulator with the aid of controllight in accordance with the information respectively to be displayed,and a signal light source for providing the signal light which is to bemodulated by the light modulator in order to visualize the informationto be displayed, the light modulator being arranged in the control lightbeam path and in the signal light beam path in such a way that thecontrol light and the signal light enter the photochromic layer on theside of the photochromic layer averted from the reflecting filter layer,the modulated signal light being capable of emerging on the side of thelight modulator opposite the light entry side. The visual observation ofthe displayed information or the modulated signal light is performedfrom the side of the light modulator averted from the light entry side.The information displayed can be a contrast image of an object maskarranged in the control light beam path which is projected onto thephotochromic layer of the light modulator.

In accordance with a preferred variant of the optical display device,the control light source is a laser, a deflecting device, in particulara biaxial scanner, being provided for the controlled deflection of thecontrol light beam, for the purpose of planar addressing, and anintensity modulator being provided which controls the intensity of thecontrol light beam as a function of its impingement position on thephotochromic layer in accordance- with the information to be displayed.

The laser can preferably be switched between two wavelengths λ_(S) andλ_(L) which are selected such that the photochromic layer can be writtenwith light of wavelength λ_(S) and can be erased with light ofwavelength λ_(L). In this context, writing the photochromic layer meansthat the photochromic layer is activated in order to change its opticalproperties for the signal light. Erasing the photochromic layer means inthis context that the photochromic layer is returned to its originalstate again.

The invention also relates to an optical arrangement having a controllight source, a signal light source and a light modulator as claimed inclaim 7 for the purpose of transmitting information contained in acontrol light beam onto a signal light beam, in which the control lightbeam carrying the respective information enters the photochromic layerfrom the side of the photochromic layer of the light modulator avertedfrom the filter layer reflecting the control light in awavelength-selective fashion, and the signal light beam enters thephotochromic layer from the side of the photochromic layer averted fromthe filter layer reflecting the signal light in a wavelength-selectivefashion.

Such an optical arrangement can be used as an incoherent/coherentconverter when the control light is incoherent and the signal light iscoherent. The information contained in the incoherent control light beamcan be transmitted in such an arrangement onto the coherent signal lightbeam. The information to be transmitted can be, for example, the imageof an object mask located in the control light beam path, which image isprojected onto the photochromic layer of the light modulator.

A further possibility of use for the above-named optical arrangementrelates to the transmission of the object information contained in acontrol light beam with the wavelength λ₁ onto a signal light beam withthe wavelength λ₂. In this case, the control light source is a laserwith the wavelength λ₁, and the signal light source a laser with thewavelength λ₂.

Preferred applications of the invention are:

(i) high-resolution optical display systems which can be viewed with anaked eye from the light exit side of an OASLM for the modulated signallight without running the risk of being dazzled and/or injured by theintense laser light which is used as control light,

(ii) projection displays for high-resolution data projection,

(iii) incoherent/coherent converters for various optical systems, and

(iv) variable masks for the exposure of light-sensitive layers inphotolithography.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are explained below withreference to the figures, in which:

FIG. 1 shows a possible configuration of a beam path for the use of anoptically addressable spatial light modulator having signal light, orprimary light, and control light, or secondary light,

FIG. 2 shows a beam path in which the light modulator functions as avisual display element,

FIG. 3 shows the basic components of the light modulator used in thearrangement of FIG. 2,

FIG. 4 shows a beam path in which the light modulator functions as anincoherent/coherent converter,

FIG. 5 shows the design of an embodiment of the light modulator which isadvantageous for the application shown in FIG. 4, and

FIG. 6 shows the beam path of a frequency converter in which the lightmodulator serves as a coherent/coherent converter with the aid of whichone wavelength can be controlled by another.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Illustrated in FIG. 1 is an optically addressable light modulator 1,denoted below as OASLM, which is struck by unmodulated signal light 2 a.The optical properties of the photochromic layer of the OASLM 1, forexample the absorption and reflectivity, or the refractive index, aremodulated spatially by the control light 3. This spatial modulation ofthe optical properties leads to a corresponding modulation of the signallight 2 a as it passes through the photochromic layer, as a result ofwhich the signal light 2 b emerging from the OASLM 1 is correspondinglymodulated. In the example in accordance with FIG. 1, the control light 3a strikes the OASLM 1 from the same side as does the unmodulated signallight 2 a. It is also possible to make simultaneous use of severalcontrol light sources (multiple modulation). Moreover, a further lightsource can be added for the purpose of photochemical erasure of theOASLM.

FIG. 2 illustrates an example of application for an OASLM 1 according tothe invention which is shown in FIG. 3 and explained in further detailbelow. FIG. 2 is a schematic of a high-resolution display system inwhich the OASLM 1 forms a display element to be viewed with the nakedeye 4. The OASLM 1, which contains bacteriorhodopsin, is illuminatedwith unmodulated signal light which emerges from the halogen lamp 5 aslight source and passes through the condenser 6, the color filter 7 anda diffuser 8. The control light is directed onto the OASLM 1 from alaser 9 via an optical system comprising a fast intensity modulator 10,a biaxial mirror scanner 11 and a suitable deflecting mirror 12, thisbeing done, specifically, from the same side as the signal light. Inorder to implement a large deflection angle for the control light beamit is possible to provide as a further component a so-called f-thetalens (not -shown) in the arrangement according to FIG. 2. The purpose ofthe intensity modulator is to control the control light intensity as afunction of the respective location of the control light beam on theOASLM 1 as determined by the desired display, a control device (notshown) controlling the mirror scanner 11 in order to move the controllight beam and the intensity modulator 10 to set the control lightintensity in each case. The mirror scanner 11 is preferably controlledsuch that the control light beam-scans the OASLM with a specificrepetition rate, doing so by row or by column over the entire lateralsurface of the OASLM, in order to “write” the information to bedisplayed to the photochromic layer (rasterized control). Alternatively,the mirror scanner 11 can be controlled such that the control light beamin each case draws only the track of the information to be displayedonto the photochromic layer (vector control). The spectrum of the signallight irradiated into the OASLM 1 comprises a wavelength region whichcorresponds to the spectral region in which the absorption behavior ofthe OASLM 1 can be influenced by the control light. Thus, in accordancewith the photochemical properties of the BR material, the control lightleads to a change or setting of the absorption behavior of thephotochromic layer as determined by the intensity pattern of the controllight, and thus to a corresponding modulation of the signal light whichstrikes an observer 4. It is also possible at 4 in FIG. 2, for example,to provide a photodetector, a light-sensitive film, applied to aworkpiece or the like, for example, in order to evaluate the modulatedsignal light. A filter layer which retroreflects the control light,after passage through the photochromic layer, in a wavelength-selectivefashion ensures that the control light is not passed—or in any event ispassed in a strongly attenuated fashion—to the side of the observer 4.In order to remove any possible residual component of control lightpassed by the OASLM 1, a linear polarization filter can be connecteddownstream of the OASLM 1 (in crossed position relative to the directionof polarization of the control light, which is linearly polarized inthis example).

A preferred application of the invention is therefore a display whichcan be observed directly with the human eye, can be “written” by acomputer-controlled laser scanner with high resolution at repetitionrates of a few seconds with the aid of the control light, and in thecase of which there is no risk at all of the human observer beingdazzled by transmitted laser light (=control light), here acomparatively strong laser, or even of exposing the observer to the riskof damage to his visual apparatus. The laser 9 can preferably beswitched back and forth between two wavelengths. The first serves to“write” the OASLM 1, while the second permits specific “erasure” of theOASLM 1. The OASLM is preferably provided with a combination of highlyreflecting dielectric layers such that both light of writing wavelengthand light of erasing wavelength is transmitted only insubstantially,that is to say by less than 5%. Such a display permits therepresentation of image information and character information with aresolution which is currently not achieved approximately by classicalcomputer display screens. Typically, the useful resolution of thedisplay according to the invention corresponds to that of laserprinters. Fields of application for such high-resolution displays arearchitecture, medicine, engineering and a multiplicity of othertechnical areas of application in which data are to be represented andviewed at very high resolution without making a paper printout for thesepurposes.

The basic components of the OASLM 1 used in the arrangement according toFIG. 2 is illustrated in detail in FIG. 3. A BR layer 15 is applied to atransparent substrate 13 with a wavelength-selectively highly reflectingcoating 14 (reflection filter 14) for the wavelength of the controllight, if appropriate the wavelength of the writing light and of theerasing light. Such wavelength-selectively highly reflective coatedsubstrates can be produced by means of known deposition methods. Suchmirrors are commercially available, for example under the designation ofhighly reflective laser mirrors, for a range of wavelength regions. TheBR layer 15 is provided here with a transparent cover layer 16 whichserves the purpose of achieving the necessary optical planeness, ofachieving a high scratch resistance, and of encapsulating the BR layeragainst the environment, that is to say fluctuating relative airhumidities etc., and can also be equipped for additional filterfunctions, if appropriate. The reflectivity of the coating 14 acting asreflection filter is at least 80%, preferably at least 90%, withparticular preference at least 99% for one or two selected wavelengths.The spectral bandwidth of the reflection filter is typically 100 nm,preferably at most 70 nm, and with particular preference less than 50 nmfor each of the selected wavelengths. The reflective layers are producedin a way known per se by deposition of the substrate with layers ofsuitable thickness and different refractive indices. If a reflectinglayer is provided both for writing light and for erasing light, thesecan be combined. The substrate 13 consists of a material which does notabsorb or absorbs only slightly in the wavelength region of visiblelight (400-800 nm), for example glass, quartz or transparent plastic.The control light (modulation light) is retroreflected by the reflectionfilter 14, passes the BR layer 15 for a second time and therebycontributes to improving the modulation. This holds both for the writingoperation and for the erasing operation. Furthermore, by comparison withthe use of an absorptive filter there is the advantage that no localheat development occurs which would, for example, lead to variations inthe refractive index of the surrounding air in the case of a coherentbeam path. At the same time, the control light is kept away from themodulated signal beam path downstream of the OASLM 1.

The BR layer 15 in direct contact with the reflection filter 14 cancomprise a BR wild type or a BR variant or a mixture of one or more BRvariants and, if appropriate, a BR wild type. The BR layer can furthercontain polymers and other auxiliaries, for example bonding agents, forlayer formation and fixing and further auxiliaries or additives forstabilizing the photon availability, the water content, the pH value andfor the purpose of adapting the refractive index. The preparation of theBR layer used here is achieved, for example, by distributing an aqueousmixture of 3% gelatin or polyvinyl alcohol and 8% bacteriorhodopsin onthe prepared substrate. After vaporization of the water, there remains afilm composed of bacteriorhodopsin and the matrix material which has athickness of less than 1 mm, typically approximately 50 μm to 250 μm.The photoproperties of the immobilized BR can be modified by amultiplicity of auxiliaries known in the literature.

A further preferred exemplary embodiment of an OASLM 1 according to theinvention is illustrated in FIG. 4. On the mutually averted sides of thephotochromic layer 15 a, the OASLM 1 according to FIG. 4 in each casehas a wavelength-selectively reflecting filter layer 14 a and 14 b. Thewavelength-selectively reflecting filter layers 14 a and 14 b havedifferent reflection characteristics, the filter layer 14 a beingprovided for wavelength-selective reflection of the control light 28 a,and the filter layer 14 b being provided for the wavelength-selectivereflection of signal light 28 b. The reflecting filter layer 14 a isapplied to the optically transparent substrate 13 a. In a correspondingway, the reflecting filter layer 14 b is located on the transparentsubstrate 13 b. The OASLM 1 can optionally have broadband antireflectioncoatings or wavelength-selective antireflection coatings 17 a and 17 bon the outsides, in order to reduce reflection losses. The photochromiclayer 15 a preferably contains bacteriorhodopsin as photochromicmaterial, which can interact both with wavelengths of the control light28 a and with those of the signal light 28 b.

Of particular technical interest is the simultaneous application oflight of a first wavelength which is preferably absorbed by the B state(for example 568 nm) and light of a second wavelength which ispreferably absorbed by the M state (for example 413 nm). Bothwavelengths can be generated using krypton gas lasers, for example.

The “air gaps”, illustrated in FIGS. 3 and 4, between the elements havebeen drawn in only for reasons of clarity of representation. They arenormally not present in the implemented embodiment.

FIG. 5 illustrates a possible application of an OASLM according to theinvention, as illustrated in FIG. 4.

FIG. 5 shows a schematic illustration of an optical design which can beused as an incoherent/coherent converter. Located on the left-hand sideof the OASLM 1 in FIG. 5 is the optical device 18-22 for generating anincoherent control light bundle 28 a which carries the information to betransmitted by the mediation of the OASLM 1. In the example, this device18-22 for generating the control light bundle 28 a comprises a halogenlamp 18 as light source, a condenser 19, a color filter 20 which passesthe desired control light wavelength, at object structure 21 and animaging optical system 22. The control light bundle 28 a thereforeprojects the information image of the object structure 21 onto the OASLM1, in order to activate the photochromic layer 15 a in accordance withthis information image. The filter layer 14 a of the OASLM 1, whichfilter layer is not illustrated in FIG. 5 but can be seen in FIG. 4,retroreflects the component of the control light 28 a passed by thephotochromic layer 15 a, such that this control light component cancontribute to the optical activation of the photochromic layer 15 aduring the second passage through the photochromic layer 15 a. Moreover,the reflecting filter layer 14 a ensures that the OASLM 1 does not pass,or in any event passes with strong attenuation, the control light intothe spatial region of the signal light situated in FIG. 4 on theright-hand side of the OASLM 1. The object structure 21 can comprise,for example, a static mask or a dynamic mask, for example in the form ofa liquid crystal display screen which is transirradiated by the controllight bundle 28 a, in order to modulate the object information onto theincoherent control light bundle 28 a. The OASLM 1 is irradiated from theright-hand side of the OASLM 1 in FIG. 5 with non-modulated coherentlight 28 b (signal light), which is passed to the photochromic layer 15a by the reflecting filter layer 14 a (FIG. 4) and is retroreflectedafter passing the photochromic layer 15 a at the filter layer 14 b,which reflects the signal light 28 b in a wavelength-selective fashion.The reflected coherent signal light 28 b′ is modulated or loaded withthe object information of the incoherent control light 28 a.

The function of the OASLM 1 according to FIG. 4 is not limited to theincoherent/coherent conversion in the sense of the arrangement accordingto FIG. 5; it is also possible to use coherent and/or incoherent lighton both sides of the OASLM 1. In each case, the photochromic BR layer isused to transmit the information from the control light beam path ontothe signal light beam path.

In the case of the OASLM 1 according to FIG. 4, the light 28 apreviously denoted as control light can take over the function of thesignal light, if the light 28 b previously denoted as signal light takesover the function of the control light. Since it is therefore possibleto drive the OASLM 1 according to FIG. 4 alternately-from the left-handside and from the right-hand side with the aid of control light, inorder to transmit the information contained in the control light via theOASLM 1 to the signal light on the respective other side, the OASLM-1can be used as a bidirectional optical data transmission element orinformation transmission element.

The arrangement according to FIG. 5 can also be operated in principlewith the aid of an OASLM 1 having the structure in accordance with FIG.3, the incoherent light striking the BR layer 15 from the side of thesubstrate 13, whereas the coherent light to be modulated is irradiatedfrom the opposite side. In this case, the mirror layer 14 would be afilter layer reflecting the coherent light (signal light). In the caseof such an incoherent/coherent converter having a light modulatoraccording to FIG. 3, however, interfering incoherent light would alsopossibly occur in the beam path of the coherent light. In order toprevent such a situation, it would be possible to operate usingpolarized incoherent light and to block out by means of polarizationfilters, for example, the incoherent light passed by the lightmodulator.

FIG. 6 illustrates a further possible application of the OASLM 1according to FIG. 4. The OASLM 1 is illuminated by a first laser 34 witha beam which is expanded by means of lenses 35 and is modulated, inintensity, by a neutral density filter 36 and modulated by a static orvariable mask 37 serving as object. The object information of the mask37 is projected onto the OASLM 1 with the aid of the imaging opticalsystem 38. In addition, a λ/4 plate 33 is provided which makescircularly polarized light from the linearly polarized light of thelaser 34. The information impressed by means of the mask 37 istransmitted into the OASLM 1 by the control light emanating from thelaser 34. The control light component penetrating the photochromic layer15 a is retroreflected at the wavelength-selectively reflecting filterlayer 14 a (FIG. 5).

Emanating from a second laser 29 is a second expanded beam (signal lightbeam) which is directed onto the OASLM 1 in unmodulated form via amirror 30 and a polarizing beam splitter 31, specifically on the side ofthe OASLM 1 opposite the control light entry side. Before the modulatedsignal light beam strikes the OASLM 1, it passes the λ/4 plate 32. Afterretroreflection at the filter layer 14 b reflecting the signal light ina wavelength-selective fashion (FIG. 5) the signal light beam has thedirection of polarization which is perpendicular to the originaldirection of irradiation polarization, and will pass through thepolarizing beam splitter. The emerging signal light beam 39 has thewavelength of the second laser 29, but carries the information of theobject mask 37 transmitted with the control light into the OASLM. Theoptical arrangement according to FIG. 6 permits the information to bemodulated from a first light beam of wavelength λ1 onto a second lightbeam of wavelength λ2. The term frequency converter is therefore used.

A particular advantage of the OASLM is yielded by the use ofbacteriorhodopsin (BR). Owing to the efficient photochemistry of the BR,it is possible to implement a very quickly operating dynamic system inthe case of which the information to be transmitted can vary in time inthe millisecond range. In the case of an application of 568 nm and 413nm for the control light and signal light, respectively, (or viceversa), a maximum amplitude modulation is achieved in simultaneousconjunction with high temporal dynamics of the OASLM, since writing anderasing can be implemented photochemically at any location.

What is claimed is:
 1. A light modulator element for modulating signallight by means of laser radiation as control light which is to be fed toa control light entry side of the light modulator element, comprising aphotochromic layer (15; 15 a) which can be optically activated by thecontrol light for the purpose of modulating signal light, and anoptically transparent substrate (13; 13 a) for the photochromic layer(15; 15 a), characterized in that the light modulator element (1) has asfurther layer on the side of the photochromic layer (15; 15 a) avertedfrom the control light entry side a reflective filter layer (14; 14 a)which is transparent to signal light but reflects the control light in awavelength-selective fashion with a spectral bandwidth of at most 70 nm,in order to retroreflect into the photochromic layer (15; 15 a) controllight which has penetrated the photochromic layer (15; 15 a), thereflective filter layer (14; 14 a) having a reflectivity as regards thecontrol light of at least 80%.
 2. The light modulator as claimed inclaim 1, in which the reflective filter layer (14; 14 a) is arrangedbetween the photochromic layer (15; 15 a) and the substrate (13; 13 a)and makes direct contact with the photochromic layer (15; 15 a).
 3. Thelight modulator as claimed in claim 1, in which the reflective filterlayer (14; 14 a) has a reflectivity of at least 99% as regards thecontrol light.
 4. The light modulator as claimed in claim 3, in whichthe reflective filter layer (14; 14 a) has a reflectivity of at least80%, in particular of at least 99%, in at least one further wavelengthregion in addition.
 5. The light modulator as claimed in claim 1, inwhich the photochromic layer (15; 15 a) contains bacteriorhodopsin. 6.The light modulator as claimed in claim 5, in which the photochromiclayer (15; 15 a) contains a variant of the wild form of thebacteriorhodopsin, which has a higher light sensitivity and/or a longerservice life of the longest lived intermediates than the wild form, inparticular a variant in the case of which the amino acid position 85 ismodified, or a variant in the case of which the amino acid position 96is modified, or a variant in the case of which dihydroretinal or4-ketoretinal serves as chromophore group, or a variant in the case ofwhich both dihydroretinal or 4-ketoretinal serve as chromophore groupand the amino acid position 85 and/or 96 are modified.
 7. The lightmodulator as claimed in claim 1, characterized in that a reflectivefilter layer (14 b) which reflects the signal light in awavelength-selective fashion is provided on the side of the photochromiclayer (15 a) averted from the reflective filter layer (14 a), in orderto retroreflect signal light which has penetrated the photochromic layer(15 a).
 8. An optical display device having a light modulator (1) asclaimed in claim 1 as display element, a laser as control light source(5) for activating the photochromic layer (15) of the light modulatorelement (1) with the aid of control light in accordance with theinformation respectively to be displayed, and a signal light source (9)for providing the signal light which is to be modulated by the lightmodulator element (1) in order to visualize the information to bedisplayed, the light modulator element (1) being arranged in the controllight beam path and in the signal light beam path in such a way that thecontrol light and the signal light enter the photochromic layer (15) onthe side of the photochromic layer (15) averted from the reflectivefilter layer (14), the modulated signal light being capable of emergingon the side of the light modulator element (1) opposite the light entryside.
 9. The optical display device as claimed in claim 8, a deflectingdevice (11), in particular a biaxial scanner, being provided for thecontrolled deflection of the control light beam, and an intensitymodulator (10) being provided which controls the intensity of thecontrol light beam as a function of its impingement position on thephotochromic layer (15) in accordance with the information to bedisplayed.
 10. The optical display device as claimed in claim 9, inwhich the laser (9) can be switched over between two wavelengths λ_(S)and λ_(L) which are selected such that the photochromic layer (15) canbe written with light of wavelength λ_(S) and can be erased with lightof wavelength λ_(L).
 11. An optical arrangement having a control lightsource (18; 34), a signal light source (29) and a light modulatorelement as claimed in claim 7 for the purpose of transmittinginformation contained in a control light beam onto a signal light beam,in which the control light beam carrying the respective informationenters the photochromic layer (15 a) from the side of the photochromiclayer (15 a) averted from the reflective filter layer (14 a) reflectingthe control light in a wavelength-selective fashion, and the signallight beam enters the photochromic layer (15 a) from the side of thephotochromic layer (15 a) averted from the reflective filter layer (14b) reflecting the signal light in a wavelength-selective fashion. 12.The optical arrangement as claimed in claim 11, in which the controllight (28 a) is incoherent and the signal light (28 b) is coherent. 13.The optical arrangement as claimed in claim 11, in which the controllight source (34) is a laser with a wavelength λ₁, and the signal lightsource (29) is a laser with the wavelength λ₂.